The mastering process all begins with mixing, as there are several steps you can take when mixing your track to make for easier, cleaner, better mastering. You should do these whether you plan to master material yourself, or hand your project to a mastering engineer.
It’s a good idea to use the same audio settings in your DAW as the recorded samples in your track when doing a mixdown. For example, if you’re using 24bit samples you should mix using 24bit settings. It’s also important to export your song using the same settings as you’re recording in. Maintain the higher resolution (24bit) throughout the mastering process, and only dither down to 16-bit at the very end. Generally you will always get a 16bit verion as your final product. DO NOT dither individual mixes, and don’t add any fades while mixing — leave all of that for mastering.
Another consideration involves the possible need for noise reduction. Sometimes there may be a slight hiss, hum, or other constant noise at a very low level. If you can obtain a clean sample of this sound, it can be loaded into a noise-reduction program that mathematically subtracts the noise from the track (also called De-Essing). Even if this noise is way down in level, removing it can improve the sound in a subtle way by opening up the sound stage and improving stereo separation. Also be sure that any processing is kept to individual channels when mixing, and not the master output channel. A lot of artists I know will mix with the limiter on the master channel giving the illusion of the track sounding bigger and better but I find that it really messes with your ability to properly mix elements in your track. Processing completed mixdowns is best left for mastering.
As you mix, you should also watch closely for distortion — a few overloads may not be audible as you listen to the mix, but may be accentuated if you add EQ or limiting while mastering. I notice this a lot in the low end mostly in the 60hz-200hz range. Listen specifically to the tail end of your kicks as there’s sometimes some distortion that will come up in volume during the limiting process in mastering. You can really tell with a good pair of headphones. I usually make some eq tweaks to my kicks whenever doing a mixdown to get rid of any unwanted frequencies.
It’s better to concede a few decibels of headroom rather than risk distortion. It’s not necessarily a good idea to add normalization when exporting, as that means another stage of DSP (Digital Signal Processing) — and you may need to change the overall level anyway when assembling all the mixes into a finished album. Go with my advice and do not use normalization during export.
And lastly, it is always best to make absolutely sure that you are 100% completely satisfied with your mixdown before sending it off to have mastered. Sometimes artists will feel the need to send a new mixdown once the mastering process has already begun. This is essentially counterproductive for both artist and mastering engineer and will save both of you time if you are sure you are satisfied with your mixdown before having it mastered.